git-gui: add mingw specific startup wrapper

git-gui: add mingw specific startup wrapper

The wrapper adds the directory it is installed in to PATH.
This is required for the git commands implemented in shell.
git-gui fails to launch them if PATH is not modified.

The wrapper script also accepts an optional command line
switch '--working-dir <dir>' and changes to <dir> before
launching the actual git-gui. This is required to implement
the "Git Gui Here" Explorer shell extension.

As a last step the original git-gui script is launched,
which is expected to be located in the same directory
under the name git-gui.tcl.

Signed-off-by: Steffen Prohaska <prohaska@zib.de>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>